1. Field of the Invention
The present invention relates generally to resonant power converters, and relates more particularly to resonant power converters that are controlled with a phase delay control configuration.
2. Description of the Related Art
Many types of power converters are well known, in particular pulse width modulation (PWM) converters and resonant power converters. PWM power converters operate by providing a pulse train, where the pulse width is adjusted according to the desired power to be supplied. PWM converters can typically switch at frequencies that provide increased efficiency, to permit a size reduction for the magnetic components, leading to smaller packaging. Typically, however, higher frequency switching in PWM converters results in increased switching losses and greater electromagnetic interference (EMI) being produced. Typically, the switching losses occur because the switches are controlled to switch while conducting a current or bearing a voltage, resulting in “hard switching.” The hard switching losses in a typical PWM converter tend to increase with the switching frequency. In addition, the EMI generated by hard switching, especially at high frequencies, can become a major factor that affects the efficiency of an input power supply through a reduced power factor.
To overcome the difficulties associated with hard switching in PWM converters, resonant converters have been used that have oscillatory waveforms that permit “soft switching,” where either the current or voltage carried by a switch is close to zero. In particular, the switches in a resonant converter can turn on with zero current and turn off with zero voltage. The reduced switching losses and simplicity of implementation permits resonant converters to operate at typically much higher frequencies than is practical with PWM converters. Accordingly, a typical resonant converter can provide a great deal of efficiency with a high power density. In addition, the oscillatory nature of the input in a resonant converter permits a control scheme to shape the input current to match that of the voltage, resulting in a high power factor. A desired power output from a resonant converter is typically controlled by changing the switching frequency to regulate the output voltage. A typical series resonant inverter is illustrated in FIG. 1, using a half bridge switching configuration in which the switches are operated complementary with regard to switching ON and OFF.
Resonant converters can be operated in a number of modes, including conductive, capacitive and resistive. FIG. 2 illustrates operational waveforms for an inductive mode of operation of the resonant converter depicted in FIG. 1. FIG. 3 illustrates operational waveforms for a capacitive mode of operation for the resonant converter depicted in FIG. 1. FIG. 4 illustrates the operational waveforms for a resistive mode of operation of the resonant converter depicted in FIG. 1.
Referring to FIG. 3, the capacitive mode of operation shows a decreased switching frequency that is lower than that of the resonant frequency for the circuit. In the capacitive mode, the body diodes of the MOSFET switches reverse recover with significant losses. Accordingly, it is preferred that the resonant converter operates at frequencies greater than the resonant frequency of the circuit to minimize these losses.
When the resonant converter is operating in resistive mode, the operation frequency is close to the resonant frequency, and thus obtains a high degree of efficiency. In this instance, the voltage and current sinusoidal waveforms have nearly the same phase, resulting in a high power factor and less energy dissipated in circulating voltages and currents. However, the operation frequency of the resonant converter must be maintained when exposed to varying loads, to continue to obtain high efficiency and a good power factor correction.
Various topologies are used in resonant converters to obtain various desired results. For example, FIG. 5 illustrates a parallel resonant converter, while FIG. 6 illustrates an LCC resonant converter. In FIG. 5, capacitor CP is the only resonant capacitor, as capacitors Cin/2 act as voltage dividers for the input DC voltage. In FIG. 6, both capacitors CP and capacitors Cs/2 act as resonant capacitors.
Operational characteristics vary among the topologies of the resonant converters described above. For example, the series resonant converter illustrated in FIG. 1 can operate in an open circuit mode, but not in a short circuit mode. The parallel resonant converter illustrated in FIG. 5 can operate in a short circuit mode, but not in an open circuit mode. The LCC resonant converter illustrated in FIG. 6, cannot operate in either short circuit or open circuit modes, and therefore preferably includes open and short circuit protection in practical operation. However, the LCC resonant converter has an increased overall efficiency and available output load range. The increased range and efficiency results from a decreased circulating current with a decreased load, so that an overall high efficiency range is maintained.
In the resonant converters described above, output voltage is typically maintained and regulated as a function of switching frequency. An increase in the switching frequency permits greater power to be delivered to the load, thereby permitting an increased power output. However, this type of control can result in resonant currents and voltages that have high peak values, which leads to increased conduction losses as well as increased rating requirements for the power devices. In addition, variable switching frequency control typically makes the overall control more complicated, as well as adding to the complexity of filter design for the converter. This type of control typically relies on feedback from the output to regulate the switching frequency and maintain the desired power output level. However, the relationship between the output power and switching frequency is typically very non-linear, adding to the difficulty of realizing a robust control for the resonant power converter.